1. Field of the Invention
The present invention relates to a method and an apparatus for friction agitation welding.
2. Description of Related Art
Robot controllers for spot welding have a function of monitoring abnormalities of robot operation to prevent an accident by forcibly terminating the robot operation at a lapse of a specified time for which an arcing command is present from an occurrence of an irregular stop of the robot. Such a robot controller is known from, for example, U.S. Pat. No. 6,377,869 B1.
In the meantime, it has been known in the art to use a friction agitation process as a joint welding process or a surface finishing process. In the friction agitation process, rotational force must be exerted on a workpiece while urging a rotating tool and the workpiece together. While the tool is rotated and advanced to apply urging pressure and rotational force to the workpiece, friction heat is generated and to plasticize the workpiece material. As a result, the tool is allowed to penetrate the workpiece, so as thereby to cause a plastic flow of the workpiece material in the workpiece. In the case where the friction agitation process is used to weld superposed workpieces together, a spot weld is formed across a joint between the superposed workpieces resulting from a plastic flow occurring at an interface between the workpieces. On the other hand, in the case where the friction agitation process is used to finish a workpiece surface, the a plastic flow makes a surface texture of the workpiece dense, increasing mechanical strength of the workpiece.
In the friction agitation process, the rotating tool is unavoidably penetrated into a workpiece. When the rotating tool is urgently stopped due to an emergency of some kind while the rotating tool remain penetrated in the workpiece, the tool is locked in a solidified workiece material. This leads to causing damages of the tool and the workpiece if removing the tool from the workpiece with the strong hand.